The present invention relates to a flow rate measuring apparatus for measuring a flow rate of a gas, etc., by intermittently driving ultrasonic transducers and/or a thermal flow sensor.
Conventionally, flow rate measuring apparatuses of the type using an ultrasonic wave are known in the art, such as that disclosed in Japanese Laid-Open Publication No. 9-21667. As illustrated in FIG. 12, in a conventional flow rate measuring apparatus, ultrasonic transducers 2 and 3 are provided as part of a fluid pipe 1 so as to oppose each other, and the ultrasonic transducer 3 is provided so as to be ahead of the ultrasonic transducer 2 in a flow direction. The flow rate measuring apparatus causes the transducer 2 to generate an ultrasonic wave along the flow direction. When the transducer 3 detects the ultrasonic wave, the flow rate measuring apparatus causes the transducer 2 to generate the ultrasonic wave again and repeats this operation to measure the duration. The flow rate measuring apparatus also causes the transducer 3 to generate an ultrasonic wave against the flow direction and similarly repeats the operation to measure the duration. The flow rate measuring apparatus calculates a fluid velocity based on a difference in the duration. In the case where such a conventional flow rate measuring apparatus is driven by batteries, if a great deal of electric power is consumed by transmitting/receiving the ultrasonic wave, the batteries run out in a short period of time. Therefore, in order to reduce power consumption, measuring operations are required to be intermittently performed based on a constant cycle or a variable cycle in accordance with a measured flow rate. In particular, when a flow rate is zero, a measurement cycle is long. In such a case, a large flow rate can unexpectedly occur, and thus a search measurement (the number of times the search measurement is repeated is less than the number of times the regular measurement is repeated) is performed in the intervals of a regular measurement so as to detect the unexpected large flow rate.
However, not only in the case where there is a zero flow rate but also in the case where there is some flow rate amount, if the measurement cycle is too long, when abrupt flow rate variations occur during an idling period of the measurement, the flow rate measuring apparatus occasionally cannot sense the abrupt variations, so that a difference in measurement is caused.
The present invention solves the above-described problem. An objective of the present invention is to reduce power consumption and realize high-precision measurement.
A flow rate measuring apparatus of the present invention includes: a flow rate detection section provided in a fluid flow path; a measuring section for measuring an output from the flow rate detection section; a cycle setting section for setting a measurement cycle of the flow rate detection section; a resolution setting section for setting a measurement resolution of the measuring section; a flow rate calculation section for calculating a flow rate based on an output from the measuring section; and a measurement control section for controlling each of the sections, in which the measurement control section includes: a regular measuring section for setting a predetermined value in the cycle setting section and the resolution setting section so as to obtain a measured flow rate based on a value obtained by the setting; and a search measuring section for setting in the cycle setting section a cycle which is shorter than that set by the regular measuring section during an idling period of the regular measuring section and for setting a coarse resolution in the resolution setting section so as to estimate a flow rate during an idling period of the regular measuring section based on a value obtained by the setting.
With the above-described structure, flow rate variations can be detected by performing the search measurement, and thus the frequency of the use of the regular measurement can be reduced. The electric power consumed by the search measurement is small and there is no need to use an additional flow rate detection section having another structure, so that the power consumption can be reduced and high-precision measurement can be realized.
The flow rate detection section may include a first transducer for transmitting an ultrasonic wave signal and a second transducer for receiving the ultrasonic wave signal, the measuring section may repeat an ultrasonic wave propagation between the first transducer and the second transducer so as to measure an accumulated time of the ultrasonic wave propagation, the cycle setting section may set a measurement start cycle for the first transducer and the second transducer, and the resolution setting section may set the number of times to repeat the ultrasonic wave propagation.
With the above-described structure, in the search measurement, the number of times to repeat the ultrasonic wave propagation can be reduced. In this case, the duration of the search measurement is shorter than that of the regular measurement and the search measurement is performed in an extremely instantaneous manner. Therefore, for example, fluctuations in a pulsed flow rate caused by fluctuations in a fluid pressure can be detected and it is possible to increase the measurement sensitivity to flow rate variations.
The flow rate, detection section may include a thermal flow sensor, the measuring section may measure heat output from the flow sensor, the cycle setting section may set a measurement cycle of the flow sensor, and the resolution setting section may set the quantity of heat input to the flow sensor so as to set a measurement resolution of the measuring section.
With the above-described structure, in the search measurement, the quantity of heat input to a heater can be reduced. The search measurement is performed such that the quantity of consumed heat is reduced in comparison to the case where the regular measurement is performed, and thus it is possible to increase the durability of hot wires of the thermal flow sensor and the reliability of a thermal flow rate measuring apparatus including such a thermal flow sensor.
The regular measuring section may set the measurement cycle in the cycle setting section so as to last for a long period of time as the measured flow rate is decreased. In this case, by performing the search measurement, flow rate variations during the idling period of the regular measurement can be sensed, and in particular, when the flow rate is small, the measurement cycle can be set so as to last for a long period of time, so that the frequency of operation can be reduced, and thus the power consumption can be reduced.
The regular measuring section may set a measurement cycle in the cycle setting section so as to last for a long period of time as an estimated flow rate is decreased. In this case, by performing the search measurement, the flow rate variations can be predicted and the regular measurement cycle can be changed, and thus, in particular, when the flow rate is small, the power consumption can be reduced and it is possible to increase the measurement sensitivity to flow rate variations.
The regular measuring section may set a measurement resolution in the resolution setting section to be coarse as an estimated flow rate is increased. Flow rate variations can be predicted by performing the search measurement. In particular, when the flow rate is large, the measurement resolution is set to be coarse, so that and an operating current is reduced. Thus, the power consumption can be reduced.
When a difference between the measured flow rate and the estimated flow rate is less than a predetermined value, the regular measuring section may set the measurement cycle in the cycle setting section so as to last for a long period of time. In this case, by performing the search measurement, when the flow rate variations are small, the measurement cycle can be set so as to last for a long period of time, so that the frequency of operation can be reduced, and thus the power consumption can be reduced.
Moreover, when a difference between the measured flow rate and the estimated flow rate is less than a predetermined value, the regular measuring section may set the measurement resolution in the resolution setting section to be coarse. In this case, by performing the search measurement, when the flow rate variations are small, the measurement resolution can be set to be coarse, thus reducing an operating current. Thus, the power consumption can be reduced.
When a difference between the measured flow rate and the estimated flow rate is equal to or more than a predetermined value, the regular measuring section may set the measurement resolution in the resolution setting section to be fine. In this case, by performing the search measurement, when the flow rate variations are large, the measurement resolution can be set to be fine, and thus the measurement sensitivity to flow rate variations is increased.
Moreover, when a difference between the measured flow rate and the estimated flow rate is equal to or more than a predetermined value, the regular measuring section may set the measurement cycle in the cycle setting section to be short. In this case, by performing the search measurement, when the flow rate variations are large, the measurement cycle can be set to be short, and thus the measurement sensitivity to flow rate variations is increased.
The flow rate measuring apparatus of the present invention may further include an integrating section for performing an integrating process using only the measured flow rate. In the case where flow rate variations obtained by the search measurement are large, the regular measurement is performed and values obtained by the regular measurement are integrated. Thus, a high-precision integrated flow rate can be obtained.
The flow rate measuring apparatus of the present invention may further include an integrating section for performing an integrating process using the measured flow rate and an estimated flow rate, in which a difference between the measured flow rate and the estimated flow rate is equal to or more than a predetermined value. In the case where flow rate variations are large, the regular measurement and integration are performed, and an estimated flow rate at a starting point of variations is reflected in an integrated value, and thus a high-precision integrated flow rate can be obtained with high measurement sensitivity to flow rate variations.